Thixotropic, thermosetting resins for use in a material extrusion process in additive manufacturing

ABSTRACT

Methods of preparing a three-dimensional structure are provided. One method includes the steps of extruding beads of thixotropic thermoset materials, and subjecting the beads to curing conditions such that the thixotropic thermoset materials at least partially cure to form cured polymer layers. In some cases, the curing conditions are not applied until multiple beads are extruded and in contact with one another. The steps of these methods can be performed repeatedly as desired to prepare a three-dimensional structure of nearly limitless shapes by additive manufacturing processes. Thixotropic thermoset materials are also provided, as are three-dimensional objects formed therefrom.

CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. patent application Ser. No.14/952,999, filed 26 Nov. 2015, which claims the benefit of U.S.Provisional Application No. 62/085,316, filed 27 Nov. 2014, thedisclosures of which are hereby incorporated herein by reference as iffully set forth below.

FIELD OF THE INVENTION

The present invention is directed to additive manufacturing and the useof thermoset resins in additive manufacturing processes.

BACKGROUND OF THE INVENTION

Additive manufacturing has been used for many years. Fabricated partshave been produced using various printing techniques (e.g.,three-dimensional or 3-D printing techniques). For example, sheetwelding, wire welding, melting in powder beds or powder deposition vialaser and electron beam melting, injections using powder, liquidultraviolet curable resins, and fusible thermoplastic filaments have allbeen used. These techniques have varying degrees of geometriccomplexity, but generally have few restrictions in comparison toconventional machining. Each type of technique has associated with itadvantages and disadvantages, particularly with respect to solid stateprocessing, fine grain structures, and mechanical properties.

Selective laser sintering (SLS) is a powder-based layer additivemanufacturing process in which laser beams, either continuous or pulsemode, are used as a heat source for scanning and joining powders inpredetermined sizes and shapes of layers via a polymer binder. Thegeometry of the scanned layers corresponds to the various cross-sectionsof the computer-aided design (CAD) models. A drawback of SLS is thatadditional powder at the boundaries is often hardened and remainsattached to the part, thereby requiring additional finishing steps toremove the unwanted material. Furthermore, an inert atmosphere is oftenrequired, increasing the cost of the equipment.

Other known processes of additive manufacturing are based on a fluidicor liquid resin layer that is selectively solidified to construct a partlayer-by-layer. One such process is known as stereolithography (SLA).SLA uses a vat of liquid ultraviolet curable photopolymer “resin” and anultraviolet laser to build parts' layers one at a time. The laser beamtraces a cross-section of the part pattern on the surface of the liquidresin. The UV exposure cures and solidifies the pattern traced on theresin and joins it to the layer below. After the pattern has beentraced, an elevator platform supporting the vat containing the resindescends a distance equal to the thickness of the single layer. A newlayer of liquid resin then forms over the part to form a new liquidsurface. A subsequent layer pattern is then traced, joining the previouslayer. This process is repeated to form a 3-dimensional part. Thecompleted part is washed in a chemical bath to remove excess resin. Thepart is then photocured in an ultraviolet oven. Although SLA can be usedto prepare parts having a variety of different shapes, the ultravioletcurable photopolymer resin can be quite expensive, and due to thecomplexity of the SLA equipment, the machines costs may be prohibitivelyexpensive. Further, photocuring is limited in terms of thickness of thepart that can be cured due to the photon gradient parallel to thedirection of the radiation source that arises from the absorption of theradiation that must occur to effect curing.

Material extrusion is another additive manufacturing process thatutilizes a fluid resin to build a part layer-by-layer. In this process,a part is formed by the extrusion of small beads of a moltenthermoplastic material that fuse to each other to form layers of thepart. The molten thermoplastic material hardens by cooling below itsmelting temperature or glass transition temperature after extrusion froma mobile nozzle. Typically, the nozzle heats the thermoplastic materialabove its glass transition temperature, and for crystalline orsemi-crystalline material, above the melting point. The molten materialis then deposited by an extrusion head. Examples of thermoplasticmaterials that are used in material extrusion include,acrylonitrile-butadiene-styrene (ABS), polylactic acid, polycarbonate,polyamides, polystyrene, and lignin. Material extrusion was firstdeveloped by Stratays, Inc. and is also known under the trademark FUSEDDEPOSITION MODELING™.

While material extrusion generally provides an effective process forbuilding a part, it does have some disadvantages. First, prior to addinga successive layer, the previously built layer needs to sufficientlycool and solidify. Second, the part may have lower strength in theZ-direction (e.g., between successive layers) due to poor entanglementof polymer chains between successive layers. In addition, for manythermoplastics, such as ABS, it is necessary to first dry the resinprior to extrusion.

Accordingly, a need still exists for new resins and processes to be usedin additive manufacturing.

SUMMARY OF THE INVENTION

In embodiments, the subject matter disclosed herein is directed tomethods of preparing three-dimensional structures. More specifically,embodiments of the invention are directed to methods of manufacturingparts in which successive layers of a thixotropic thermoset material aresuccessively deposited onto one another to form a part.

The thixotropic thermoset material comprises a thermoset resin to whicha rheology control agent has been added. The rheology control agentrenders the thermoset resin thixotropic so that the material is capableof being extruded through a nozzle to form a bead on a surface thatretains its shape upon being deposited and does not flow afterdeposition on the surface at the point where mechanical shear stressesare removed.

As a result, successive layers of the thixotropic thermoset material maybe deposited essentially without any change in geometry. When curingconditions are applied after successive layers have been deposited, thepolymer chains of adjacent contacting layers crosslink with each otherduring a curing step to form a part having improved strength in thez-direction of the part.

In contrast, many prior art methods of additive manufacturing, such asthose utilizing thermoplastics, require that a preceding layer issufficiently cool and hardened prior to depositing a subsequent layer.As a result, entanglement of the polymer chains between adjacent layersis non-existent or limited at best, which in return results in lowerstrength in the z-direction of the manufactured part.

One method comprises i. extruding a first bead of a first thixotropicthermoset material onto a support, wherein the first thixotropicthermoset material comprises a first thermoset resin and a firstrheology control agent; ii. subjecting the first bead to curingconditions such that the thixotropic thermoset material is at leastpartially cured to form a cured first polymer layer; iii. extruding asecond bead of a second thixotropic thermoset material in contact withthe cured first polymer layer, wherein the second thixotropic thermosetmaterial comprises a second thermoset resin and a second rheologycontrol agent; and iv. subjecting the second bead of thixotropicthermoset material to curing conditions, wherein the second bead ofthixotropic thermoset material is at least partially cured to form acured second polymer layer, and wherein the three-dimensional structureis prepared.

The steps of the method can be performed repeatedly as desired toprepare a three-dimensional structure of nearly limitless shapes.

In one embodiment of such a method, the first and/or second thixotropicthermoset material has a thixotropic index that is greater than 5.Specifically, the first and/or second thixotropic thermoset material canhave a thixotropic index that is 10 or higher. More specifically, thefirst and/or second thixotropic thermoset material can have athixotropic index that is 15 or higher. Even more specifically, thefirst and/or second thixotropic thermoset material can have athixotropic index that is 20 or higher. Yet more specifically, the firstand/or second thixotropic thermoset material can have a thixotropicindex that is 25 or higher.

In the same or another embodiment of such a method, the first and/orsecond thermoset resin is selected from the group consisting of phenolicresins; lignin resins; tannin resins; amino resins; polyimide resins;isocyanate resins; (meth)acrylate resins; vinylic resins; styrenicresins; polyester resins; melamine resins; vinyl ester resins; maleimideresins; epoxy resins; polyamidoamine resins; and mixtures thereof. Morespecifically, the first and/or second thermoset resin can be selectedfrom the group consisting of phenolic resins, amino resins, epoxyresins, isocyanate resins, and acrylate resins.

In the same or another embodiment of such a method, the phenolic resincan have a mole ratio of formaldehyde to phenol of about 2:1 to about3:1.

In the same or another embodiment of such a method, the phenolic resincan have a crosslinker and a 0.6 to 0.9 ratio of formaldehyde to phenol.

In the same or another embodiment of such a method, the amino resins canbe resins having a mole ratio of formaldehyde to urea from about 2.2:1to about 3.8:1.

In the same or another embodiment of such a method, the first curedpolymer layer is cross-linked with the second cured polymer layer.

In the same or another embodiment of such a method, the first and/orsecond thixotropic thermoset material is capable of flowing whensubjected to an external shear stress and at zero shear rate having ayield strength or yield point such that the first and/or secondthixotropic thermoset material does not flow.

In the same or another embodiment of such a method, the steps ofsubjecting the first or second beads to curing conditions compriseirradiating the first or second bead with thermal energy.

In the same or another embodiment of such a method, the steps ofsubjecting the first or second bead to curing conditions comprisesubjecting the first or second bead to visible or invisible light,UV-radiation, IR-radiation, electron beam radiation, X-ray radiation orlaser radiation.

In the same or another embodiment of such a method, the first and/orsecond rheology control agent comprises fumed silica, organoclays,polysaccharides, cellulose and derivatives thereof.

In the same or another embodiment of such a method, the steps ofextruding a first or second bead of the first or second thixotropicthermoset material comprise subjecting the first or second thixotropicthermoset material to an external shear stress to cause the first orsecond thixotropic thermoset material to be extruded through anextrusion nozzle.

In the same or another embodiment of such a method, the firstthixotropic thermoset material has the same composition as the secondthixotropic thermoset material.

In the same or another embodiment of such a method, the first and/orsecond thixotropic thermoset material is extruded through a heatednozzle that initiates curing of the first and/or second thixotropicthermoset material.

In the same or another embodiment of such a method, steps ii. and iv.result in the cured first polymer layer and cured second polymer layer,respectively, being only partially cured to allow for subsequentcrosslinking between the first and second layers.

Another method comprises: i. extruding a first bead of a firstthixotropic thermoset material onto a support, wherein the firstthixotropic thermoset material comprises a first thermoset resin and afirst rheology control agent, and wherein the first thixotropicthermoset material has a thixotropic index that is greater than 5; ii.extruding a second bead of a second thixotropic thermoset material,wherein the second bead is in contact with the first bead, wherein thesecond thixotropic thermoset material comprises a second thermoset resinand a second rheology control agent, and wherein the second thixotropicthermoset material has a thixotropic index that is greater than 5; andiii. subjecting the first and second beads to curing conditions to formcured first and second polymer layers, respectively, wherein the curedfirst polymer layer is cross-linked with the cured second polymer layer,and wherein the three-dimensional structure is prepared.

In one embodiment of such a method, the method further includessuccessively repeating steps i. and ii, prior to step iii to form thethree-dimensional structure comprising a plurality of cured polymerlayers, wherein adjacent cured polymer layers are cross-linked with eachother.

In the same or another embodiment of such a method, the step ofsubjecting the first and second beads to curing conditions comprisesheating the first and second beads to a temperature ranging from about25 to about 125° C.

In the same or another embodiment of such a method, the first and/orsecond thermoset resin is selected from the group consisting of phenolicresins; lignin resins; tannin resins; amino resins; polyimide resins;isocyanate resins; (meth)acrylate resins; vinylic resins; styrenicresins; polyester resins; melamine resins; vinyl ester resins; maleimideresins; epoxy resins; polyamidoamine resins; and mixtures thereof. Morespecifically, the first and/or second thermoset resin can be selectedfrom the group consisting of phenolic resins, amino resins, epoxyresins, isocyanate resins, and acrylate resins.

In the same or another embodiment of such a method, the phenolic resincan have a mole ratio of formaldehyde to phenol of about 2:1 to about3:1.

In the same or another embodiment of such a method, the phenolic resincan have a crosslinker and a 0.6 to 0.9 ratio of formaldehyde to phenol.

In the same or another embodiment of such a method, the amino resins canbe resins having a mole ratio of formaldehyde to urea from about 2.2:1to about 3.8:1.

In the same or another embodiment of such a method, the first and/orsecond thixotropic thermoset material is capable of flowing whensubjected to an external shear stress and at zero shear rate having ayield strength or yield point such that the first and/or secondthixotropic thermoset material does not flow.

In the same or another embodiment of such a method, the steps ofsubjecting the first or second beads to curing conditions compriseirradiating the first or second bead with thermal energy.

In the same or another embodiment of such a method, the steps ofsubjecting the first or second bead to curing conditions comprisesubjecting the first or second bead to visible or invisible light,UV-radiation, IR-radiation, electron beam radiation, X-ray radiation orlaser radiation.

In the same or another embodiment of such a method, the first and/orsecond rheology control agent comprises fumed silica, organoclays,polysaccharides, cellulose and derivatives thereof.

In the same or another embodiment of such a method, the steps ofextruding a first or second bead of the first or second thixotropicthermoset material comprise subjecting the first or second thixotropicthermoset material to an external shear stress to cause the first orsecond thixotropic thermoset material to be extruded through anextrusion nozzle.

In the same or another embodiment of such a method, the firstthixotropic thermoset material has the same composition as the secondthixotropic thermoset material.

In the same or another embodiment of such a method, the first and/orsecond thixotropic thermoset material is extruded through a heatednozzle that initiates curing of the first and/or second thixotropicthermoset material.

One thixotropic thermoset material comprises a thermoset resin and arheology control agent, wherein the thixotropic thermoset material iscapable of flowing when subjected to an external shear stress andexhibits little to no lateral flow when in a static state, and whereinthe thixotropic thermoset material has a thixotropic index that isgreater than 5.

In one embodiment of such a material, the thermoset resin is selectedfrom the group consisting of phenolic resins; lignin resins; tanninresins; amino resins; polyimide resins; isocyanate resins;(meth)acrylate resins; vinylic resins; styrenic resins; polyesterresins; melamine resins; vinyl ester resins; maleimide resins; epoxyresins; polyamidoamine resins; and mixtures thereof. More specifically,the thermoset resin can be selected from the group consisting ofphenolic resins, amino resins, epoxy resins, isocyanate resins, andacrylate resins.

In the same or another embodiment of such a method, the phenolic resincan have a mole ratio of formaldehyde to phenol of about 2:1 to about3:1.

In the same or another embodiment of such a method, the phenolic resincan have a crosslinker and a 0.6 to 0.9 ratio of formaldehyde to phenol.

In the same or another embodiment of such a method, the amino resins canbe resins having a mole ratio of formaldehyde to urea from about 2.2:1to about 3.8:1.

In the same or another embodiment of such a material, rheology controlagent comprises fumed silica, organoclays, polysaccharides, celluloseand derivatives thereof.

The material can be used to form a three-dimensional object.

One three-dimensional object comprises a plurality of layers each builtat least partially on top of another, and in which each layer defines across section of the three-dimensional object, and wherein each layercomprises a cured polymeric material in which a polymer chain of a givenlayer is crosslinked with a polymer chain of an adjoining layer.

In one embodiment of such an object, the cured polymeric material isderived from a thixotropic thermoset material. The thixotropic thermosetmaterial can, as defined above, comprise a thermoset resin and arheology control agent. The thixotropic thermoset material can becapable of flowing when subjected to an external shear stress andexhibits little to no lateral flow when in a static state, and thethixotropic thermoset material can have a thixotropic index that isgreater than 5.

In the same or another embodiment of such an object, thethree-dimensional object can include four or more layers. Specifically,the three-dimensional object can include between 2 and 10,000 layers.More specifically, the three-dimensional object can include 100 to 500layers.

These and other aspects of the subject matter are disclosed in moredetail in the description of the invention given below.

DETAILED DESCRIPTION OF THE INVENTION

The presently disclosed subject matter will now be described more fullyhereinafter. However, many modifications and other embodiments of thepresently disclosed subject matter set forth herein will come to mind toone skilled in the art to which the presently disclosed subject matterpertains having the benefit of the teachings presented in the foregoingdescriptions. Therefore, it is to be understood that the presentlydisclosed subject matter is not to be limited to the specificembodiments disclosed and that modifications and other embodiments areintended to be included within the scope of the appended claims.

Disclosed herein, are advantageous processes and materials for use inadditive manufacturing processes in which a thixotropic thermosetmaterial comprising a thermoset resin and at least one rheology controlagent is deposited on a surface as a plurality of beads to form one ormore layers of a three-dimensional structure. The thermoset material iscured to provide a three-dimensional structure.

As discussed in greater detail below, the thermoset material includes arheology control agent to provide a thermoset material havingthixotropic properties. As a result, the thixotropic thermoset materialis capable of being extruded through a nozzle to form a bead on asurface that retains its shape upon being deposited and does not flowafter deposition on the surface.

The use of a thermoset resin provides advantages in comparison to theuse of a molten thermoplastic material. In particular, a moltenthermoplastic material, such as that used in conventional materialextrusion, gains strength as the material cools. As a consequence, theresulting cured article may not have uniform strength throughout itsstructure. In comparison, the present invention provides a method inwhich the thixotropic thermoset material may be cured after one or morebeads, or one or more layers have been deposited in contact with eachother. Such delayed curing may allow for crosslinking between adjacentbeads and adjacent layers to thereby produce a three dimensionalstructure having improved strength.

Definitions

As used herein the term “additive manufacturing” refers to any processof joining materials to make objects by depositing layer upon depositedlayer. Each layer will have the desired dimensions and shape such thattogether the layers form a three-dimensional, engineered structure.

As used herein, the term “thermoset” or “thermosetting” refers to aproperty of a polymer precursor or polymer made from such precursorwhere the polymer once crosslinked is irreversibly cured such that afterthermosetting has taken place, the resin cannot be melted or dissolvedwithout some chemical decomposition taking place first. The cure may beinduced by heat above the set temperature, through a chemical reactionthat leads to formation of covalent bonds that were not present prior tocure. In addition to heat, some thermoset polymers may be cured via achemical reaction in which two components chemically react to cure thepolymer. Other methods of curing may include exposure to a humidenvironment, such as a chamber having a relative high humidity.

As used herein, the term “thermoset resin” or “thermosetting resin”refers to precursor materials that will form a thermoset polymer wheninduced to polymerize and crosslink as described herein. Thermosetresins are distinguishable from thermoplastic materials and resins,which are known in the art. Thermosetting resins are chemically distinctfrom thermoplastic resins and can be contrasted with thermoplasticpolymers which are commonly produced in pellets and shaped into theirfinal product form by melting and pressing or injection molding.

As used herein the term “thixotropic thermoset material,” “thixotropicthermoset resin,” or “thixotropic thermosetting resin” refers to athermoset resin that has been formulated to have thixotropic propertiesby the addition of one or more rheological control agents. Thixotropicthermoset materials in accordance with embodiments of the presentinvention exhibit shear-thinning behavior when subjected to a shearstress, and at least partial recovery (increase) of viscosity uponremoval of the shear stress. As a result, the thixotropic thermosetmaterials are capable of flowing when subjected to a shear stress, andexhibit no or minimal flow in the absence of the shear stress. Morespecifically, thixotropic thermoset materials in accordance with thepresent invention when under a zero shear rate have a yield strength orBingham yield point that is greater than the force of gravity, such thatthe thixotropic material is static and does not flow without an externalshear stress greater than the force of gravity is applied. As notedpreviously, the thixotropic thermoset materials exhibit a recovery orpartial recovery of viscosity upon removal of the shear stress. Bypartial recovery it is meant that the viscosity of the materialfollowing removal of the shear stress is sufficiently increased suchthat a second bead of the thixotropic thermoset material deposited ontoa previously deposited bead of the material will retain its shape andnot flow into the previously applied bead. In other words, thethixotropic thermoset material has an initial viscosity (also referredto herein as its static viscosity) prior to the application of a shearstress, and a second viscosity that is lower than the initial viscositywhen a shear stress is applied. Upon removal of the shear stress, thematerial exhibits at least partial recovery of the initial viscosity.Ideally, thixotropic thermoset materials in accordance with embodimentsof the present invention are capable of being deposited as a bead thatexhibits little to no flow, and that retains its shape followingdeposition on a surface.

As used herein, the terms “rheological control agent,” “rheologicalcontrol additive,” “rheology control agent,” and “rheology controladditive” are used interchangeably to refer to an additive that iscombined with a thermoset resin to provide a thixotropic thermosetmaterial.

As used herein, the term “curing” refers to the chemical crosslinkingwithin the resin and between different layers of resin. Other chemicalchanges may be occurring at the same time that crosslinking isoccurring. The term “crosslinking” refers to the formation of covalentbonds between thermoset resin monomers, oligomers or polymers andpolymers formed therefrom. Such chemical changes are distinguished froma physical change such as melting. In thermoset polymers, unlikethermoplastic polymers, the curing is considered irreversible. Curingand the term “cure” refer to “partial” or “full” curing. As used herein,the term “partial” or “partially” cure, cured or curing refers to anamount of chemical crosslinking within the resin and between differentlayers of resin to form covalent bonds between the resin molecules andlayers. As used herein, the term “full” or “fully” cure, cured or curingrefers to an amount of chemical crosslinking within the resin andbetween different layers of resin to form covalent bonds between theresin molecules and layers such that subjecting the resin to additionalcuring conditions does not provide appreciably more of the same type ofcovalent bonding. Accordingly, the term “fully” does not imply that allof the crosslinking moieties must be covalently bonded.

A polymer layer that is “cured” refers to a polymer layer in which atleast a portion of the available reactive sites within the polymer havereacted to form crosslinks between polymer chains in the layer or withpolymer chains in adjacent layers (i.e., partially or fully cured, asdefined above). Accordingly, as used herein, a “cured” polymer includesthose materials which are at least partially cured.

The term “structure” or “three-dimensional structure” and the like asused herein refer generally to intended or actually fabricatedthree-dimensional configurations, objects, or parts that are fabricatedand intended to be used for a particular purpose. Such structures, etc.may, for example, be designed with the aid of a three-dimensional CADsystem. The shapes are engineered, meaning that they are particularshapes designed and manufactured according to specification in thedesired shape as contrasted with random shapes. The structures will becomprised of layers as described herein. In contrast, structures formedfrom other methods, such as molding, will not contain such layers. A“plurality” of structures refers to two or more of such structures thatare substantially identical. As used herein, the term “substantially”implies that the structures are identical in all respects but areallowed to have minor topological imperfections. Additionally, the terms“three-dimensional model” and “3D model” refer to objects, parts, andthe like built using layer-based additive manufacturing techniques, andare not intended to be limited to any particular use.

The terms “about” and “substantially” are used herein with respect tomeasurable values and ranges due to expected variations known to thoseskilled in the art (e.g., limitations and variabilities inmeasurements).

As used herein, the term “bead” refers to a continuous stream or line ofresin that is deposited on a surface by at least one extrusion nozzle.The bead may be linear or non-linear, and may have a variety ofcross-sections including circular, elliptical, rectilinear, trapezoidalor other shapes.

As used herein a “single layer” of resin can be any amount of materialapplied in any fashion prior to the addition of any new material (thenext layer) proximate to a previously deposited layer of material.Typically, a single layer of material is provided proximate to asubstrate or a previously deposited layer of material. A single layermay comprise a single bead of extruded resin or a plurality of beads ofextruded resin that have been cured or partially cured. Preferably,curing of the layers occurs after two or more layers have been depositedso that polymer chains of adjoining layers (e.g., layers built one ontop of each other) crosslink with each other, and thereby improve thestrength of the resulting part in the in z-direction.

As used herein, the term “contacting” includes extruding, applying,spreading, filling dumping, dropping and the like such that the beads ofthe thermoset resin are in position for the processes described hereinto proceed.

As used herein, the term “curing conditions” refers to conditions underwhich the resin cures. Types of curing conditions include thermal energy(e.g., radiative heating), humidity, and chemical reactions betweenmulticomponent systems.

As used herein, the term “irradiation” refers to the thermal energy theresin is subjected to such that the resin is heated through radiativeheating. As discussed elsewhere herein, irradiation may be achieved by alaser source, oven, or the like. The rate and amount of irradiation canvary depending on the parameters of the additive manufacturing processesand devices utilized. In one embodiment, irradiation does not includephotocuring of the resin.

Thixotropic Thermoset Material

Thixotropic thermoset materials in accordance with embodiments of thepresent invention comprise a thermoset resin and a rheological controlagent. As discussed above, the addition of the rheological control agenthelps to modify the rheological properties of the thermoset resin andthereby render it thixotropic. The thixotropic thermoset material maythen be dispensed from an extrusion nozzle as a bead that thereafterretains its shape until cured with minimal or no lateral flow. Afterdeposition, the thixotropic thermoset resin may be cured using curingmethods known in the art.

Thixotropic thermoset materials in accordance with embodiments of thepresent invention may have a thixotropic index that is greater than 5.The thixotropic index is a ratio of a material's viscosity at two shearrates. A thixotropic material's viscosity will reduce as the shear ratethrough agitation is increased. This index indicates the thixotropy ofthe inventive thixotropic thermoset material. In the present invention,the thixotropic index is the ratio of the viscosity of the resin at 0.1sec⁻¹ to the viscosity of the resin at 1 sec⁻¹ both measured at 25° C.

In general, the only practical upper limit on the thixotropic index ofthe thixotropic thermoset resin is the ability of the apparatus (e.g., apump) to cause the material to flow under shear stress. Preferably, thethixotropic index of the thixotropic thermoset material is greater than6, and more preferably greater than 10, and even more preferably,greater than 25.

Useful thermoset resins for use in embodiments disclosed herein are anyknown thermoset resins that are in the form of a liquid, and arecompatible with a rheological control agent to form a thixotropicthermoset resin.

Examples of suitable classes of thermoset resins may include phenolicresins, amino resins, redox curing monomeric acrylates,isocyanatoururethanes, polyisocyanates, epoxies, olefin-containingresins, and polyamidoamine-epichlorohydrin adducts. In particular,useful thermoset resin may be selected from the group consisting ofthermoset phenolic resins; thermoset cyanoacrylate resins, thermosetlignin resins; thermoset tannin resins; thermoset amino resins;thermoset polyimide resins; thermoset isocyanate resins; thermoset(meth)acrylic resins; thermoset Maillard reactants, thermoset vinylicresins; thermoset styrenic resins; thermoset polyester resins; thermosetmelamine resins; thermoset vinyl ester resins; thermoset maleimideresins, such as bismaleimide resins; thermoset cyanate ester resins;epoxy resins; polyamidoamine resins; and mixtures thereof.

One useful phenolic, thermosetting resin is REST-BOND® 6773 sold byGeorgia-Pacific Chemicals. A phenolic, curable precursor resin can rangein mole ratio of formaldehyde to phenol from 2:1 to 2.95:1. A morepreferred range is from 2:1 to 2.65:1. The pH of a phenolic, curableprecursor resin can range from 7.1 to 13.9. A more preferred range isfrom 8.5 to 12.9. A phenolic, curable precursor resin at 50 wt % solidsin an aqueous medium can range in viscosity from 60 to 60,000 cps atroom temperature. A more preferred range for the phenolic, curableprecursor resin at 50 wt % solids in an aqueous medium is from 100 cpsto 2000 cps at room temperature.

Another category of thermoset resins include polyamidoamines andpolyamidoamine-epihalohydrin adducts. An example of a suitablepolyamidoamine-epihalohydrin adduct resin is AMRES® 1110-E availablefrom Georgia-Pacific Chemicals. Examples of polyamidoamines andpolyamidoamine-epihalohydrin adduct resins that may be useful in someembodiments of the present invention are described U.S. Pat. Nos.2,926,154, 3,086,961, 3,700,623, 3,772,076, 4,233,417, 4,298,639,4,298,715, 4,341,887, 4,853,431, 5,019,606, 5,510,004, 5,644,021,6,429,267, 7,189,307, and 8,785,593.

One example of an amino resin is a urea-formaldehyde resin sold underthe trade name GP® 600D16 by Georgia-Pacific Chemicals.

In one embodiment, the thermoset resin may be non-aqueous. Examples ofnon-aqueous thermoset resins include acrylates resins, such as methylmethacrylate and butylacrylate, isocyanate resins, and thermoset epoxyresins.

The thermoset resin may also be selected from thermoset resins known inthe art including at least one resin selected from polyimide resins;isocyanate resins; (meth)acrylic resins; phenolic resins; vinylicresins; styrenic resins; polyester resins; melamine resins; vinyl esterresins; maleimide resins; and mixtures thereof.

The resins that have been named are intended to be examples of classeswithout limiting the range of materials that may be used.

The resins described herein, including those known in the art, cancontain a catalyst. The type of catalyst will be chosen based on thecrosslinking moieties on the thermosetting resin and is well within theskill of those in this field. Non-limiting examples include thefollowing. For example, the crosslinking of isocyanate moieties can becatalyzed with dimethylaminopyridine or dibutyl tin oxide. Thecrosslinking of resole phenol formaldehyde resins can be catalyzed withsodium hydroxide, potassium hydroxide or salts ofethylenediamine-sulfonic acid. The crosslinking of a methacrylate resinis initiated by a redox initiator consisting of, for example, cumenehydroperoxide and dimethylaniline.

The amount of rheology control additive that is blended with thethermoset resin generally depends on the desired cure and flowproperties of the prepared thixotropic thermoset resin. In general, theamount of rheology control additive is selected to provide a thixotropicthermoset material as defined previously, and that exhibits little to nolateral flow or movement when under zero externally applied shearstress. However, when subjected to a sufficient externally applied shearstress, the thixotropic thermoset material will undergo shear thinningso that the resin will flow, and is capable of being deposited from anextrusion nozzle as a bead. As noted previously, the thixotropicthermoset material also exhibits recovery or partial recovery of itsinitial viscosity upon removal of the shear stress.

In one embodiment, the thixotropic thermoset resin is formulated to havea change from an initial viscosity of about 1000 cps or greater when ina static or non-shear stress state and to have a viscosity of less than100 cps when subjected to shear stress. Preferably, the thixotropicthermoset resin recovers or at least partially recovers its staticviscosity upon removal of the externally applied shear stress by beingdeposited as a bead in the building process. The selection of the amountof rheology control additive to include in the formulation is wellwithin the skill of those in this field.

Examples of suitable rheology control additives may include fumed silicaorganoclays, such as bentonite clays, polysaccharides, cellulosiccompounds, such as microcrystalline cellulose, cellulose acetate andcellulosic ether, and derivatives thereof, coal tar, carbon black,textile fibers, glass particles or fibers, aramid pulp, boron fibers,carbon fibers, mineral silicates, mica, powdered quartz, hydratedaluminum oxide, wollastonite, kaolin, silica aerogel or metal powderssuch as aluminum powder or iron powder. Among these, fumed silica ispreferred.

Commercially available examples of fumed silica that may be used inembodiments of the present invention are HDK® T-30 available fromWacker, and AEROSIL® 200 available from Evonik Degussa.

In some embodiments, the rheology control additive may be hydrophobic.Generally, the amount of rheology control additive in the thixotropicthermoset material is from about 1 to 35 weight percent, based on thetotal weight of the material, and more preferably, from about 2 to 25percent, and even more preferably, from about 4 to 25 weight percent,and most preferred from about 5 to 15 weight percent, based on the totalweight of the material.

In embodiments comprising an inorganic based rheology control additive,the amount of inorganic is generally less than 50 weight percent, andmore preferably less than 35 weight percent, and even more preferablyless than 25 weight percent, based on the total weight of the material.

In addition to the thermoset resin and rheology control additive, thethixotropic thermoset material may also include additional additives.Examples of additives that may be used include curing enhancing agents,stabilizers, such as photo-stabilizers, diluents, fillers, antioxidants,viscosity modifying agents, pigments and dyes, fire-retarding agentslubricants, dispersants, impact modifiers, adhesion promoters, andcombinations thereof.

These additives are commercially available from a wide variety ofsources and are well known by those of skill in the art. One of skill inthe art would readily identify which additives are desirable dependingon the intended application and end use of the part.

For example, a wide variety of different impact modifiers may be usedincluding cycloneopentyl, cyclo (dimethylaminoethyl) pyrophosphatozirconate, dimesyl salt, acrylonitrile/methacrylonitrile copolymer,butadiene/acrylonitrile copolymer, silicone resin, and isopropyltridodecylbenzenesulfonyl titanate.

As disclosed elsewhere herein, the use of curable thermoset resins hasmany advantages. An exceptionally useful aspect of embodiments describedherein is the ability to use a thermoset resin in an additivemanufacturing process to form a 3D object of almost any shape orgeometry comprised or consisting essentially of the cured thermosetresin.

In an embodiment, the subject matter described herein is directed to amethod of preparing a three-dimensional structure, the methodcomprising:

-   -   i. extruding a bead of a thixotropic thermoset resin onto a        build platform;    -   ii. subjecting the bead of the thixotropic thermoset resin to        curing conditions, wherein the thixotropic thermoset resin is at        least partially cured to form a cured first polymer layer;    -   iii. extruding a second layer of the thixotropic thermoset resin        with the cured first polymer layer; and    -   iv. subjecting the second layer of thixotropic thermoset resin        to curing conditions, wherein the second layer of thixotropic        thermoset resin is at least partially cured to form a cured        second polymer layer, and wherein the three-dimensional        structure is prepared.

In some embodiments, the cured first polymer layer may be cross-linkedwith the cured second polymer layer.

As will be discussed in detail below, the method can further comprisedinfusing any pores in the structure with an organic or inorganicpenetrant material.

In further embodiments, the subject matter described herein is directedto a method of preparing a three-dimensional structure, the methodcomprising:

-   -   i. extruding bead of a thixotropic thermoset resin with a build        platform;    -   ii. extruding one or more additional successive beads of the        thixotropic thermoset resin in contact with the first bead of        thixotropic thermoset resin;    -   iii. subjecting the thus extruded beads of the thixotropic        thermoset resin to curing conditions to form a plurality of        cured polymer layers, wherein the cured polymer layers are        cross-linked to adjacent layers, and wherein at least one of the        layers comprises a thermoset resin; wherein the        three-dimensional structure is prepared.

Thus the structures prepared will comprise a layer of cured thermosetresin in an engineered pattern having desired dimensions and shape.

The steps may be repeated successively as many times as desired toproduce an engineered, three-dimensional structure in an additivemanufacturing technique. As described herein therefore, the process caninclude independently selecting any type of thixotropic thermoset resinin any successive step to prepare a structure having the desiredcomposition.

In certain embodiments, subjecting the thixotropic thermoset resin tocuring conditions may include subjecting the thermoset resin to thermalradiation including exposure to actinic radiation, visible or invisiblelight, UV-radiation, IR-radiation, electron beam radiation, X-rayradiation or laser radiation in order to heat and cure the resin. In oneembodiment, subjecting the thixotropic thermoset resin to curingconditions may comprise exposing the structure to thermal energy fromheating elements, as in an oven. In other embodiments, the step ofsubjecting the thixotropic thermoset resin to curing conditions maycomprise exposing the structure to a high relative humidity. Forexample, certain thermoset resins having isocyanate moieties may undergocuring by being exposed to moisture. In other embodiments, thesubjecting the resin to curing conditions may comprise mixing of twochemical components to effect a chemical reaction that results in curingof the resin. For example, the reaction of an oxidant such as cumenehydroperoxide with a reductant such as dimethyl aniline to form a redoxinitiator to effect the cure of methyl methacrylate.

In practicing the disclosed methods, a first layer of thixotropicthermoset resin can be the same type of resin as the second layer ofthermoset resin. As a non-limiting example, both layers can be the samespecies of thermoset phenolic resin. Alternatively, the first layer ofthixotropic thermoset resin can be a different type of resin than thesecond layer of thermoset resin. As a non-limiting example, one layercan be a thermoset phenolic resin and the second layer can be athermoset amino resin; or one layer can be a phenolic resin species andthe other layer a different species of phenolic resin. Accordingly,successive layers in the structure can be composed of the same ordifferent materials.

In embodiments, the present subject matter is directed to a structurecomprising, a cured thermoset resin having an engineeredthree-dimensional shape. The structure will comprise one or more layersof a cured thermoset resin. Accordingly, in embodiments, the structurecan contain from 2 to an unlimited number of engineered layers; from 2to about 10,000 layers; from 2 to about 5,000 layers; from 2 to about1,000 layers; from 2 to about 500 layers; from 2 to about 250 layers;from 2 to about 100 layers; from 10 to about 500 layers; from 50 toabout 500 layers; from 100 to about 500 layers; or from 250 to about 500layers. Each layer may be of the same or different type of resin. Eachlayer may be the same or different dimensions. There is almost no limitto the shapes that can be prepared by additive manufacturing. The shapeswill be designed and engineered to a specification. The methodsdescribed herein can prepare the structures according to thespecification. In embodiments, the structure is an engineeredthree-dimensional shape designed using computer-aided design. Almostunlimited substantially identical copies of the structures can beprepared by the methods. In aspects of this embodiment, the presentsubject matter is directed to a plurality of monodispersethree-dimensional structures comprising, two or more discretestructures, each comprising or consisting essentially of a curedthermoset resin having an engineered three-dimensional shape, whereineach structure of the plurality is substantially identical. In thisembodiment, the material distinction is that the structure is fabricatedusing essentially only the thixotropic thermoset resin.

However, the structures may contain other components. In another aspect,any pores in the structure formed by curing the thixotropic thermosetresin may be infused with organic material or inorganic material.Accordingly, the method of preparing a structure as described above canfurther comprise contacting the formed structure with an organic orinorganic material to infuse any pores in the structure with the organicor inorganic material. In this embodiment, the step of contacting caninclude immersing, soaking and the like for an amount of time sufficientto infuse. The infusion step may be performed under increased pressureto facilitate infusion.

Those of ordinary skill in the art would immediately recognize thematerials that are compatible with each other for infusion. In otherwords, the penetrant material should be chosen to be compatible with thecured material of the structure.

One organic material that may be used to infuse the structure is anepoxy resin such as Epon® 828. Other resins that may be used includepolymeric MDI, polyurethanes, and acrylic resins.

An inorganic material that can be used to infuse the structure is moltenBelmont alloy. Other molten metals that may be considered are copper,bronze, silver, tin, pewter, lead and aluminum.

Additionally, radiation energy may be used to carbonize the structureafter the thixotropic thermoset resin has cured. The carbonization stepmay be done as part of the curing process or may be done as a separatestep with the same laser or a second laser. The carbonization step maybe done in an ambient atmosphere or done in either an oxygen-richatmosphere or an inert atmosphere.

The structure from such a process may range in carbon content from about65 wt % carbon to about 99.5 wt % carbon; from about 70 wt % carbon toabout 95 wt % carbon; from about 75 wt % carbon to about 90 wt % carbon;or from about 80 wt % carbon to about 85 wt % carbon. Furthermore,carbonized materials may be dosed with one or more different materialssuch as metals to impart functionalities such as embedded catalysts onporous carbon. This type of multi-material structure can be realizedby 1) placing metal powder (or other material powder) selectively at allcalculated interception points for the currently formed carbonizedlayer, and 2) sintering or melting the powder to infuse it onto carbonlayer. More metals or other materials can be added by repeating aboveprocedure with a different powder.

Additive Manufacturing Processes and Devices

Additive manufacturing is defined by the American Society for Testingand Materials (ASTM) as the “process of joining materials to makeobjects from 3D model data, usually deposit layer upon deposit layer, asopposed to subtractive manufacturing methodologies, such as traditionalmachining and casting.” As referred herein as “additive manufacturing,”there are a number of processes for creating a digital model andproducing a three-dimensional solid object of virtually any shape fromthat model. These processes are colloquially named 3D printing, rapidprototyping, additive manufacturing, and the like.

As disclosed herein, the usefulness of additive manufacturing technologyto provide low cost product assembly and the building of any number ofproducts with engineered, complex shapes/geometries, complex materialcompositions and designed property gradients has been expanded tothixotropic thermoset resins as the material for the build for use inmaterial extrusion processes in particular.

In an additive-manufacturing process, a model, such as a design model,of the component may be defined in any suitable manner. For example, themodel may be designed with computer aided design (CAD) software. Themodel may include 3D numeric coordinates of the entire configuration ofthe component including both external and internal surfaces. The modelmay include a number of successive 2D cross-sectional slices thattogether form the 3D component.

There is typically a relatively high cost of operation and expertise isrequired to operate 3D printers. 3D design files can be created usingCAD software, such as SolidWorks™, to generate a digital representationof a 3D object. The STL (Standard Tessellation Language) file format isa commonly used format for storing such CAD files. This CAD file, inother words the digital representation of the 3D object, is subsequentlyconverted into a series of contiguous 2D cross sections, representingsequential cross-sectional slices of the 3D object. These 2D crosssections are commonly referred to as 2D contour data. The 2D contourdata can be directly input into a 3D printer in order for the printer toprint the 3D object. Conversion of a 3D design file into 2Dcross-sectional data is often carried out by dedicated software.

As such, additive manufacturing systems are used to print or otherwisebuild three-dimensional 3D parts from digital representations of the 3Dparts (e.g., AMF and STL format files) using one or more additivemanufacturing techniques. In the present invention, the additivemanufacturing techniques described herein are directed to anextrusion-based process in which a thixotropic thermoset resin isdeposited and cured in successive layers to form a 3D part.

At an initial stage, the digital representation of the 3D part is slicedinto multiple horizontal layers. For each sliced layer, a tool path isthen generated, which provides instructions for the additivemanufacturing system to print the given layer.

A three-dimensional structure of the present invention may be builtusing, for example, a three-dimensional printing system similar toembodiments described in U.S. Pat. Nos. 5,121,329, and 6,658,314.

An exemplary three-dimensional extrusion system may generally includeone or more extrusion nozzles, and at least dispenser in which thethixotropic thermoset resin is disposed. The system will may alsoinclude a pump, piston or similar device to apply external shear stressto the thixotropic thermoset resin and thereby cause the resin to flowfrom the dispenser to an associated extrusion nozzle. The thixotropicthermosetting resin may be supplied to the extrusion nozzle as a singlecomponent system, or alternatively, may include two or more componentsthat are only mixed prior to extrusion. A static mixing tube is onedevice that is designed to intimately mix two components immediatelyprior to extrusion.

In one embodiment, the extrusion system may include at least twoextrusion nozzles. For example, a first extrusion nozzle may beconnected to a first dispenser that is used to dispense a firstthixotropic thermoset resin, and a second extrusion nozzle may beconnected to a second dispenser that is used to dispense a secondthixotropic thermoset resin.

The built platform may comprise a work table, substrate or the like,which may be a releasable substrate, on which the three-dimensionalarticle is to be formed.

The three-dimensional extrusion system further includes a controller,CAD system, optional curing device, and optionally a positioningapparatus. The controller is coupled to the CAD system, curing unit,positioning apparatus, extrusion nozzle(s) and dispensers containing thethixotropic thermoset resin. Control may be effected by other unitsthan, such as one or more separate units. The three-dimensionalstructure is built in layers, the depth of each layer typically beingcontrollable by selectively adjusting the output from each extrusionnozzle.

Depending on the nature of the thermoset resin and the desired curingmechanism, the curing device may be integral to the three-dimensionalextrusion system, or may comprise a separate and stand-alone device. Forexample, the curing device may comprise an energy source for deliveringthermal energy (e.g., radiant heat) to the deposited resin wherebycuring of the thermoset resin takes place, heating element, oven, highrelative humidity chamber, or the like.

In one embodiment, the extrusion nozzle may be heated to initiate curingof the thixotropic thermoset resin as it is deposited onto the buildplatform.

The systems employed in the embodiments described herein are generallyused for manufacturing three-dimensional structures from a curablethermoset resin and for manufacturing an engineered three-dimensionalstructure comprised of the cured thermoset resin. This manufacturing canbe employed for rapid prototyping. A device comprising a source ofcuring energy (such as a CO₂ laser, IR lamp, oven, etc.) can provide thenecessary curing conditions to effect curing of the resin.

A three-dimensional structure is formed through consecutive depositionand immediate or eventual crosslinking of consecutively formed crosssections of layers, successively laid down by the extrusion nozzle.Systems for depositing a layer containing or more beads of thethixotropic thermoset resin may comprise any known means for laying outa bead of flowable resin material.

The devices utilize a computing system which implements design toolsand/or topology optimization according to desired design aspects. Thesystem includes a memory. The memory may store data. The memory maystore executable instructions used to implement the topologyoptimization according to the desired design. The executableinstructions may be stored or organized in any manner and at any levelof abstraction, such as in connection with one or more processes,routines, procedures, methods, etc.

The instructions stored in the memory may be executed by one or moreprocessors. The processor may be coupled to one or more input/output(I/O) devices. In some embodiments, the I/O device(s) may include one ormore of a keyboard or keypad, a touchscreen or touch panel, a displayscreen, a microphone, a speaker, a mouse, a button, a remote control, ajoystick, a printer, a telephone or mobile device (e.g., a smartphone),a sensor, etc. The I/O device(s) may be configured to provide aninterface to allow a user to interact with the system in the generationof a specification according to the desired build.

The specification is transferred to an additive manufacturing devicewhich performs the additive manufacturing techniques according to thespecification in order to create the 3D structure. While not required inall aspects, the additive manufacturing device can include processorsthat interpret the specification, and control other elements which applythe materials using robots, nozzles, lasers or the like to add thematerials as layers or coatings to produce the 3D structure.

The following discussion is provided as an example of how athree-dimensional structure can be built in accordance with embodimentsof the present invention. It should be recognized that other systems andprocesses can be used to build three dimensional structures using thethixotropic thermoset resins described herein.

In an initial step, the thixotropic thermoset resin is subjected tosufficient external sheer stress to cause the resin to flow from adispenser to an associated nozzle. The resin is then deposited as a beadonto the build platform. In one embodiment, the nozzle may make multiplepasses, with each pass taking place in a controlled pattern as dictatedby the computer software to form a single layer.

Curing of the thixotropic thermoset resin may be initiated while theresin is being deposited, or initiated after one or more of thedeposition of a bead, multiple beads, a single layer, multiple layers,or combination thereof is completed. In some embodiments, thethixotropic thermoset resin may be subjected to curing after each layeror after two or more successive layers have been deposited.

Generally, the computer and related software programs determine when theextrusion nozzle is on and off based on the digital computer model. Themachine controller controls the operation of the extrusion nozzle alongthe “X,” “Y,” and “Z” axes via a plurality of drive motors. Each ofthese motors may be operating separately, or one or more of them may beoperating simultaneously, depending upon the shape of the structure tobe formed. Circular patterns for each layer can be generated bycontrolled movement along the “X” and “Y” axes of the build platform.

The extrusion nozzle may be initially positioned a predetermined heightabove the build platform to form the first layer of the threedimensional structure. The height of each subsequent layer is thenclosely controlled. Typically, thinner layers provide result in thesurface of the structure having an overall smoother surface. Thickerlayers generally increase the speed at which the structure is built.Layers as thin as 0.0001 inches may be formed. The layers can be formedhorizontally, vertically, or in any 360° orientation to the horizontal.Depositing of the resin may take place along any of the three axes. Thedispensing of the resin may take place along only the “X”-“Y” plane,until it is advantageous to deposit in the “X” “Z” plane or the “Z” “Y”plane. Normally, the extrusion nozzle will be mounted along the “Z” axisgenerally perpendicular to the build platform, and thus perpendicular tothe “X”-“Y” plane of build platform.

When forming and building up multiple layers, one or more beads of thethixotropic thermoset resin are deposited to form a first layer. Thefirst layer may take any shape dictated by the computer program. Thefirst layer (or multiple layers) may then be subjected to an energysource to initiate curing of the resin. A second and each subsequentlayer may take slightly different shapes, as dictated by the particularcross section for each layer from the computer program and layeringsoftware. In the pattern situation for each layer wherein each layer isformed only in a horizontal “X”-“Y” plane. A motor supporting theextrusion nozzle may be selectively actuated after each layer is formedto move the nozzle or build platform incrementally along the “Z” axis aclosely controlled, predetermined distance to control the gap betweenlayers and thus the thickness of each layer.

After the extrusion nozzle or build platform is thus moved, the nextlayer is dispensed and formed along a controlled path. In someinstances, the extrusion nozzle may be moving in a direction along the“Z” axis as the layer is formed, such as when forming a spiral pattern,and the software program will control the location of the extrusionnozzle or build platform at the end of each layer. Thus, when at thestart position for the next layer, the extrusion nozzle or buildplatform may have already been moved a distance along the “Z” axis abovethe corresponding point on the previously-formed layer. In such asituation, the extrusion nozzle or build platform may not have to bemoved at all at the commencement of the next layer, or it may be movedincrementally a very small distance to form the desired gap betweenlayers, and thus the predetermined layer thickness.

The multiple layers may be of uniform thickness, or the layers may varyin thickness, as necessary and appropriate for the forming of aparticular structure. Also, the layers may each vary in thickness acrossthe height of each layer.

Additive manufacturing systems build the solid part one layer at a time.Typical layer thicknesses range from about 0.001-10.00 mm. However,depending on the build design, the layer may be thicker or thinner aspracticable. The thickness can be adjusted depending on the processparameters, including the average height of the bead in the layer, thetotal number of layers that make up the structure, and the speed inwhich the structure is being built.

The device may operate generally according to a method comprising thefollowing steps: (i) depositing out one or more beads of the thixotropicthermoset resin on the build platform to form a layer; (ii) curing thelayer by subjecting the deposited layer of thixotropic thermoset resinto radiation (e.g., thermal energy), (iii) laying out one or moresuccessive beads to form a successive layer on top of the previouslayer; (iv) curing the successive layer to form the next cross-sectionallayer; (v) repeating steps (iii) and (iv) until the three-dimensionalstructure is built. As noted it previously, in some embodiments, it isbe desirable to first deposit multiple successive layers prior toinitiating a step of curing the thixotropic thermoset resin.

Energy Sources

Curing of the thermoset resins may be accomplished in a variety ofdifferent ways depending on the thermoset resin. In one embodiment,curing is accomplished by subjecting the resin to irradiation thatthermally heats the resin to effect curing. In preferred embodiment,thermal energy is used for curing. Curing temperatures may typicallyrange from 25° C. to 125° C.

The electromagnetic radiation may include actinic radiation, visible orinvisible light, UV-radiation, IR-radiation, electron beam radiation,X-ray radiation, laser radiation, or the like. Moreover, while each typeof electromagnetic radiation in the electromagnetic spectrum may bediscussed generally, the disclosure is not limited to the specificexamples provided. Those of skill in the art are aware that variationson the type of electromagnetic radiation and the methods of generatingthe electromagnetic radiation may be readily determined.

The following examples are offered by way of illustration and not by wayof limitation.

EXAMPLES

In the following examples, various thixotropic thermoset materials wereprepared and evaluated based on their thixotropic properties andpotential usefulness in additive manufacturing processes.

The materials used in the thixotropic thermoset materials are identifiedbelow. All percentages are weight percents unless indicated otherwise.All physical property and compositional values are approximate unlessindicated otherwise.

-   “THR-1”, refers to a polyamidoamine-epichlorohydrin adduct resin    available under the trademark AMRES® 1110-E from Georgia-Pacific    Chemicals.-   “THR-2”, refers to phenol-formaldehyde resin available under the    trademark RESI-BOND® 6773 from Georgia-Pacific Chemicals.-   “ITU”: refers to an isocyanate-terminated urethane available under    the product name GORILLA® Glue, available from Gorilla Glue, Inc.    The ITU had a molecular weight of about 4200 performed by gel    permeation chromatography using a Waters HSPGel RT column and    detected by refractive index.-   “TEAOH” refers to a triethanolamine available from Sigma-Aldrich    sold as Triethanolamine, reagent grade.-   “MMA”: refers to a methyl methacrylate containing ≦30 ppm MEHQ,    which is available from Sigma-Aldrich.-   “BAA”: refers to a benzaldehyde-aniline adduct available under the    product name VANAX® 808, from R.T. Vanderbilt Co.-   “EPOXY” refers to a liquid epoxy resin that is a reaction product of    epichlorohydrin and bisphenol-A, and is available under the product    name EPO-KEM® 1000, from Kemrock Industries and Exports Ltd.-   “IPD”: refers to an isophone diamine available as    5-Amino-1,3,3-trimethylcyclohexanemethylamine, mixture of cis and    trans from Sigma-Aldrich.-   “CSM”: refers to a solution of chlorosulphonated polyethylene    available under the product name TOSOH CSM® from Tosoh Corp.-   “RH-1”: refers to fumed silica rheology modifier available under the    product name HDK® T-30 from Wacker.-   “RH-2”: refers to fumed silica rheology modifier available under the    product name HDK® H-18 from Wacker.-   “RH-3”: refers to fumed silica rheology modifier available under the    product name HDK® N-20 from Wacker.-   “RH-4”: refers to fumed silica rheology modifier available under the    product name HDK® H-13L from Wacker.-   “GG”: refers to a guar gum rheology modifier available from Fischer    Scientific.-   “KC”: refers to a kaolin clay rheology modifier available from    Sigma-Aldrich.

Example 1 Thixotropic Polyamidoamine-Epichlorohydrin Adduct ResinFormulation

185.5 grams of an aqueous solution of a polyamidoamine-epichlorohydrinadduct (THR-1) was weighed into a cup. The pH of the aqueous THR-1 resinwas adjusted to 6.38 by adding 50 weight percent aqueous NaOH dropwise.After the pH was adjusted, 9.6 grams of RH-1 (a fumed silica fromWacker) in 1 gram aliquots was added as a rheology control agent to theTHR-1 resin. The formulation was stirred by hand at room temperatureafter each aliquot until the RH-1 was thoroughly wetted by the aqueousresin.

A cylindrical sample of this thixotropic material with a diameter ofapproximately 1 cm was placed on a horizontal wooden tongue depressor.When the tongue depressor was rotated to the vertical position, thethixotropic material was not visually observed to move for five minutesat room temperature.

Example 2 Thixotropic Phenolic Resin Formulation

79.5 grams of a thixotropic resin of THR-2 (a phenol-formaldehyde) wasweighed into a cup. The pH of this aqueous resin was 12.7 and this wasnot adjusted. 7.0 grams of RH-1 was added in 3.5 gram increments. Theformulation was stirred by hand at room temperature after each incrementuntil the RH-1 was thoroughly wetted by the aqueous resin.

A cylindrical sample of this thixotropic formulation with a diameter ofapproximately 1 cm was placed on a horizontal wooden tongue depressor.When the tongue depressor was rotated to the vertical position, thethixotropic material was not visually observed to move for five minutesat room temperature.

Example 3 Part Made by Material Extrusion of ThixotropicPolyamidoamine-Epichlorohydrin Adduct Resin Formulation Combined withthe Material Extrusion of a Thixotropic Phenolic Resin

The formulation of thixotropic polyamidoamine-epichlorohydrin adduct(tPAE) from Example 1 was loaded into a disposable, 10 ml syringe. Theformulation of thixotropic phenolic (tPF) resin from Example 2 wasloaded into a second, disposable, 10 ml syringe. A bead of tPAE wasextruded from the nozzle of the syringe onto a borosilicate microscopeslide. The bead was about 1 mm in diameter and 2 inches long. A secondbead of tPF was extruded onto the bead of tPAE immediately after thefirst bead was applied. This second bead had the same dimensions as thefirst bead. A third bead was applied to the second bead immediatelyafter the second bead was applied. This third bead was the tPAEformulation and had the same dimensions as the first and second bead. Afourth bead was applied to the third bead immediately after the thirdbead was applied. This fourth bead was the tPF formulation and had thesame dimensions as the first, second and third bead.

After the fourth bead was applied, the microscope slide was placed in aforced air oven for 15 minutes at 105° C. The microscope slide wasremoved from the oven and allowed to cool to room temperature. Aftercuring in the oven, the part was rigid and clearly showed four distinctlayers of extruded material. The cured and cooled part exhibited was notvisually changed in any dimension from the time it was extruded on themicroscope slide.

Example 4 Thixotropic Thermoset Isocyanate Resin Formulation

69.1 grams of ITU (an isocyanate-terminated urethane resin) was weighedinto a paper cup. To this aliquot of isocyanate-terminated urethaneresin was added 5.23 grams of RH-1. The formulation was then stirred byhand at room temperature until the RH-1 was thoroughly wetted by theisocyanate-terminated urethane resin. A cylindrical sample of thisthixotropic material with a diameter of approximately 1 cm was placed ona horizontal wooden tongue depressor. When the tongue depressor wasrotated to the vertical position, the thixotropic material was notvisually observed to move for five minutes at room temperature.

Example 5 Part Made by Material Extrusion of Thixotropic IsocyanateResin Formulation

The thixotropic isocyanate resin formulation from Example 4 was placedin a 50 ml disposable syringe. A bead of the thixotropic resin with adiameter of approximately 5 mm was dispensed from the syringe onto abuild platform of aluminum foil. The first bead that was dispensed wasapproximately 4 cm long. As soon as the first bead was dispensed on thebuild platform, a second bead was immediately laid on top of the firstbead. This part was allowed to cure at room temperature for sixteenhours under ambient conditions. The cured part was not visually changedin any dimension from the time it was extruded on the build platform.

Example 6 Thixotropic, Two-Part Isocyanate-Terminated Urethane ResinFormulation

In Example 6, a two-part (Part A and Part B) thixotropic thermosetmaterial was formulated and evaluated. Part A comprised 29.48 grams ofITU prepared from methylene diphenyl diisocyanate and polypropyleneoxide. To this was added 1.45 grams of RH-1 as a rheology modifier. Theresulting mixture was mixed for 60 seconds with a hand-held homogenizer.

Part B of the two part formulation was prepared by mixing 11.03 grams ofTEAOH with 0.55 grams of RH-1. The resulting mixture was mixed for 60seconds with a hand-held homogenizer. Then 3.60 grams of part A wasmixed with 1.20 grams of part B in a syringe with the hand-heldhomogenizer.

Example 7 Part Made by Material Extrusion of a Two-Part, ThixotropicIsocyanate-Terminated Urethane Resin Formulation

Part A and part B were made as described in Example 6 above. The 4.81grams of part A was combined with 0.60 grams of part B in a syringe andmixed as described in Example 6. Then the thixotropic resin material(parts A and B combined) was extruded from the tip of the syringe toform a bead that was 1 mm in diameter and 2 cm long on a glassmicroscope slide. A second, third and fourth bead were immediatelyextruded on the preceding bead as soon as the preceding bead had beenlaid down. The resulting structure of four beads remained upright and nochange in the dimensions of the beads was observed.

Example 8 Thixotropic Methyl Methacrylate Resin Formulation

A solution of 82.5 grams of methyl methacrylate (MMA) and 9.1 grams of abenzaldehyde-aniline adduct (BAA) was made. 1.5 ml of cumenehydroperoxide was added to the solution. Then, 7.5 grams of RH-2 wasadded in small aliquots with sufficient agitation to wet out the fumedsilica after each addition. After the last addition, agitation wascontinued until the mixture was uniform by inspection. This mixture wasPart A.

Part B was then prepared by forming a solution of CSM (achlorosulphonated polyethylene) and methyl methacrylate. 46.6 grams of asolution that was 40 weight percent CSM in methyl methacrylate wascombined with 46.5 grams of MMA. Then 6.6 grams of RH-2 was added tothis solution following the above described procedure. This secondsolution was Part B.

Both Part A and Part B were allowed to stand at room temperature withoutagitation for 24 hours.

Example 9 Part Made by Material Extrusion of a Two-Part ThixotropicMethyl Methacrylate Formulation

10 grams of Part B from Example 8 was placed in the large side of a 2:1two component polypropylene application cartridge and 5 grams of Part Afrom example 8 was placed in the small side of the same cartridge. Thetwo components were mixed through a static mixer (sold by Nordson EFD)that was 5.9 inches long, contained 20 mixing elements, and had a tipdiameter of 1 mm. The exudate from the static mixer was applied to thesupporting surface in a bead 1 mm in diameter. After 1 minute, a secondbead was placed directly on the first bead. This process was repeateduntil 4 beads were extruded on top of each other. After 90 minutes atroom temperature and ambient atmosphere, the part was sufficiently curedto be handled and removed from the supporting surface. The finalthickness of the four layer construction was about 3.9 mm.

Comparative Example 1 Part Made by Material Extrusion of a Two-PartMethyl Methacrylate Formulation that Contained No Rheology Control Agent

Part A solution consisting of 20 grams of MMA and 2.2 grams of BAAwithout any rheology control agent fumed silica was made as described inexample 8. Part B solution consisting of 8 grams of the 40 weightpercent CSM solution, 8 grams of MMA, and 240 μl of cumene hydroperoxidewithout any fumed silica was made as described in example 8. These twoparts of the resin system were used to fill a cartridge and extrudedthrough a static mixer as described in example 9. Four beads of thistwo-part formulation were extruded on top of each other as described inexample 9. The final thickness of the four layer construction was about0.9 mm. This measured thickness demonstrated that the beads flowedwithout shear stress being applied thereby resulting in a thickness ofthe combined four layers of beads being less than the tip of the staticmixer.

Example 10 Thixotropic Two-Part Epoxy Resin Formulation

71 grams of an epoxy resin (EPOXY) was combined with 2.1 grams of RH-2in a plastic bottle. Four stainless steel balls were placed in thebottle and the solution was rolled at 16 rpm until the RH-2 wasuniformly dispersed. Isophorone diamine (IPD) was used to make Part B ofthis two-part resin formulation. In a second plastic bottle, 100 gramsof IPD was combined with 12 grams of RH-2. Four stainless steel ballswere placed in this second bottle and were rolled at 16 rpm until theRH-2 was uniformly dispersed.

Example 11 Part Made by Material Extrusion of a Two-Part ThixotropicEpoxy Resin Formulation

10 grams of Part A from example 10 was placed in the large side of a 2:1two component polypropylene application cartridge and 5 grams of Part Bfrom example 10 was placed in the small side of the same cartridge. Thetwo components were mixed through a static mixer that was 5.9 incheslong and contained 20 mixing elements as described in example 9. Theexudate from the static mixer was applied to the supporting surface in abead 1 mm in diameter. A second, third and fourth bead were immediatelyextruded on the preceding bead as soon as the preceding bead had beenlaid down. The structure of four beads remained upright and no change inthe dimensions of the beads was observed.

Examples 1 through 11 show that a thixotropic, thermosetting resinformulation can be extruded to form a structure of defined shape. Thecomparative example shows that, if the formulation is not thixotropic,then that formulation cannot be extruded to form a structure of definedshape.

Examples 12-26

In the following examples, various thixotropic thermoset materials wereprepared and evaluated for suitability for use in additive manufacturingprocesses. Four different resin systems were formulated with differentrheology control agents using methods similar to the procedure describedin Example 1.

The different combinations of resin system and rheology control agentsare shown in Table 1 below.

The only resin systems that required pH adjustment were thepolyamidoamine-epichlorohydrin adduct as was done in Example 1 (Examples25 and 26). The remaining resin systems were used as supplied.

The resin systems of Examples 12 to 26 were based on the resincomponents used in the above examples 1-11. More specifically, themethyl methacrylate resin system (MMA of Examples 12-14) was the sameresin components as described in Example 8; the NCO-terminated urethane(NCO-TU of Examples 15-21) was the same resin components as described inExample 6; the epoxy resin system (Epoxy of Examples 22-24) was based onthe same resin components as described in Example 10, and thepolyamidoamine-epichlorohydrin adduct system (PEA of Examples 25 and 26)was based on the same resin components as described in Example 1.

Different rheology control agents were mixed with the various resinsystems at different weight percentages to evaluate the effect on thethixotropic properties of the thixotropic thermoset materials. Eachresin system was extruded at room temperature from a 25 ml syringe witha 1 mm orifice. Four successive layers were applied such that the2^(nd), 3^(rd) and 4^(th) layers rested on each preceding layer. Aformulation was judged to pass when four layers could be appliedsuccessively without flow of the first applied bead.

In addition, the thixotropic index of each of the resin systems wasdetermined in which the rheometry was of each resin system was evaluatedwith an AR-2000ex Rheometer sold by TA Instruments. As noted previously,thixotropic index is defined as the ratio of the viscosity at 0.1 sec⁻¹to the viscosity at 1 sec⁻¹ measured at 25° C.

TABLE 1 Evaluation of Thixotropic Properties of Resin Systems FourAmount of Layers Rheology applied Rheology Control without Example ResinControl Agent Thixotropic flow No. System Agent (wt. %) Index(pass/fail) 12 MMA¹ RH-3 5.0 11.4 pass 13 MMA¹ RH-2 5.0 25.0 pass 14MMA¹ RH-4 5.0 5.0 fail 15 NCO-TU² RH-3 2.0 1.1 fail 16 NCO-TU² RH-3 2.01.8 fail 17 NCO-TU² RH-3 2.0 1.6 fail 18 NCO-TU² RH-3 5.0 0.8 fail 19NCO-TU² RH-3 5.0 3.3 fail 20 NCO-TU² RH-3 5.0 2.5 fail 21 NCO-TU² RH-34.5 10.2 pass 22 Epoxy³ RH-3 5.0 1.1 fail 23 Epoxy³ RH-3 5.0 6.0 pass 24Epoxy³ RH-3 5.0 1.8 fail 25 PEA⁴ GG 5.0 10.4 pass 26 PEA⁴ KC 5.0 1.5fail ¹Methyl methacrylate(resin material of Example 8). ²NCO-terminatedurethane (resin material of Example 6). ³Two-part epoxy resin system ofExample 6. ⁴Polyamidoamine-epichlorohydrin adduct resin system ofExample 1.

From Table 1 above, it can be seen that the thixotropic resin systemshaving a thixotropic index of 5 or less failed, and were thereforedetermined to be unsuitable for use in additive manufacturing processesof the present invention. The thixotropic materials having a thixotropicindex above 5, and in particular, at least 6 or above, all passed theevaluation.

That which is claimed:
 1. A method of preparing a three-dimensionalstructure, the method comprising: i. extruding a first bead of a firstthixotropic thermoset material onto a support, wherein the firstthixotropic thermoset material comprises a first thermoset resin and afirst rheology control agent; ii. subjecting the first bead to curingconditions such that the thixotropic thermoset material is at leastpartially cured to form a cured first polymer layer; iii. extruding asecond bead of a second thixotropic thermoset material in contact withthe cured first polymer layer, wherein the second thixotropic thermosetmaterial comprises a second thermoset resin and a second rheologycontrol agent; and iv. subjecting the second bead of thixotropicthermoset material to curing conditions, wherein the second bead ofthixotropic thermoset material is at least partially cured to form acured second polymer layer, and wherein the three-dimensional structureis prepared.
 2. The method of claim 1, wherein the first and/or secondthixotropic thermoset material has a thixotropic index that is greaterthan
 5. 3. The method of claim 1, wherein the first and/or secondthermoset resin is selected from the group consisting of phenolicresins; lignin resins; tannin resins; amino resins; polyimide resins;isocyanate resins; (meth)acrylate resins; vinylic resins; styrenicresins; polyester resins; melamine resins; vinyl ester resins; maleimideresins; epoxy resins; polyamidoamine resins; and mixtures thereof. 4.The method of claim 1, wherein the first and/or second thermoset resinis selected from the group consisting of phenolic resins, amino resins,epoxy resins, isocyanate resins, and acrylate resins.
 5. The method ofclaim 1, wherein the first cured polymer layer is cross-linked with thesecond cured polymer layer.
 6. The method of claim 1, wherein the firstand/or second thixotropic thermoset material is capable of flowing whensubjected to an external shear stress and at zero shear rate having ayield strength or yield point such that the first and/or secondthixotropic thermoset material does not flow.
 7. The method of claim 1,wherein the steps of subjecting the first or second beads to curingconditions comprise irradiating the first or second bead with thermalenergy.
 8. The method of claim 1, wherein the steps of subjecting thefirst or second bead to curing conditions comprise subjecting the firstor second bead to visible or invisible light, UV-radiation,IR-radiation, electron beam radiation, X-ray radiation or laserradiation.
 9. The method of claim 1, wherein the first and/or secondrheology control agent comprises fumed silica, organoclays,polysaccharides, cellulose and derivatives thereof.
 10. The method ofclaim 1, wherein the steps of extruding a first or second bead of thefirst or second thixotropic thermoset material comprise subjecting thefirst or second thixotropic thermoset material to an external shearstress to cause the first or second thixotropic thermoset material to beextruded through an extrusion nozzle.
 11. The method of claim 1, whereinthe first thixotropic thermoset material has the same composition as thesecond thixotropic thermoset material.
 12. The method of claim 1,wherein the first and/or second thixotropic thermoset material isextruded through a heated nozzle that initiates curing of the firstand/or second thixotropic thermoset material.
 13. A method of preparinga three-dimensional structure, the method comprising: i. extruding afirst bead of a first thixotropic thermoset material onto a support,wherein the first thixotropic thermoset material comprises a firstthermoset resin and a first rheology control agent, and wherein thefirst thixotropic thermoset material has a thixotropic index that isgreater than 5; ii. extruding a second bead of a second thixotropicthermoset material, wherein the second bead is in contact with the firstbead, wherein the second thixotropic thermoset material comprises asecond thermoset resin and a second rheology control agent, and whereinthe second thixotropic thermoset material has a thixotropic index thatis greater than 5; and iii. subjecting the first and second beads tocuring conditions to form cured first and second polymer layers,respectively, wherein the cured first polymer layer is cross-linked withthe cured second polymer layer, and wherein the three-dimensionalstructure is prepared.
 14. The method of claim 13, further comprisingsuccessively repeating steps i. and ii, prior to step iii to form thethree-dimensional structure comprising a plurality of cured polymerlayers, wherein adjacent cured polymer layers are cross-linked with eachother.
 15. The method of claim 13, wherein the step of subjecting thefirst and second beads to curing conditions comprises heating the firstand second beads to a temperature ranging from about 25 to about 125° C.16. The method of claim 13, wherein first thixotropic thermoset materialhas the same composition as the second thixotropic thermoset material.17. The method of claim 13, wherein the first and/or second thermosetresin is selected from the group consisting of phenolic resins; ligninresins; tannin resins; amino resins; polyimide resins; isocyanateresins; (meth)acrylate resins; vinylic resins; styrenic resins;polyester resins; melamine resins; vinyl ester resins; maleimide resins;epoxy resins; polyamidoamine resins; and mixtures thereof; and whereinthe first and/or second rheology control agent comprises fumed silica,organoclays, polysaccharides, cellulose and derivatives thereof.
 18. Athixotropic thermoset material comprising a thermoset resin and arheology control agent, wherein the thixotropic thermoset material iscapable of flowing when subjected to an external shear stress andexhibits little to no lateral flow when in a static state, and whereinthe thixotropic thermoset material has a thixotropic index that isgreater than
 5. 19. The thixotropic thermoset material of claim 18,wherein the thermoset resin is selected from the group consisting ofphenolic resins; lignin resins; tannin resins; amino resins; polyimideresins; isocyanate resins; (meth)acrylate resins; vinylic resins;styrenic resins; polyester resins; melamine resins; vinyl ester resins;maleimide resins; epoxy resins; polyamidoamine resins; and mixturesthereof and wherein the first and/or second rheology control agentcomprises fumed silica, organoclays, polysaccharides, cellulose andderivatives thereof.
 20. A structure comprising, one or more layers of acured thixotropic thermoset material of claim
 18. 21. Athree-dimensional object comprising a plurality of layers each built atleast partially on top of another, and in which each layer defines across section of the three-dimensional object, and wherein each layercomprises a cured polymeric material in which a polymer chain of a givenlayer is crosslinked with a polymer chain of an adjoining layer.
 22. Theobject of claim 21, wherein the cured polymeric material is derived froma thixotropic thermoset material.